A receiver, also known as User Equipment (UE), mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The receivers in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. The geographical area where radio coverage is provided by the radio network node/base station at a base station site is sometimes referred to as a cell. Cell may more generally be referred to as a logical concept transmitting radio signals without having any physical meaning. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the receivers within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the receiver to the radio network node.
Contemporary wireless systems, such as the 3GPP LTE such as e.g. Evolved Universal Terrestrial Radio Access (E-UTRA) and/or Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), utilize packet based transmissions. Upon reception of a data packet, the receiver (UE in 3GPP terminology) transmits Hybrid Automatic Repeat Request (HARQ) messages to the radio network node (eNodeB in 3GPP terminology). These messages may for example comprise an acknowledgement (ACK) or a negative ACK (NACK). New packet transmission or packet retransmissions could subsequently be initialized by the transmitter once the HARQ feedback is obtained. HARQ feedback signalling will require uplink transmission resources and it is essential to minimize the amount of time-frequency resources to be allocated for HARQ feedback since unused uplink resources could be utilized for transmitting user data instead. A further problem is to assign the uplink HARQ resources to the receiver without incurring significant signalling in the downlink and typically uplink resources may only be assigned when there is an actual data packet transmitted and HARQ feedback is expected. The assignment of the set of uplink resources also has to assure that there are no uplink resource conflicts, i.e., each receiver/UE must be assigned a set of unique uplink resources for HARQ.
In order to divide forward and reverse communication channels on the same physical communications medium, when communicating in the wireless communication system, a duplexing method may be applied such as e.g. Frequency-Division Duplexing (FDD) and/or Time-Division Duplexing (TDD). The FDD approach is used over well separated frequency bands in order to avoid interference between uplink and downlink transmissions. In TDD, uplink and downlink traffic are transmitted in the same frequency band, but in different time intervals. The uplink and downlink traffic is thus transmitted separated from each other, in the time dimension in a TDD transmission, possibly with a Guard Period (GP) in between uplink and downlink transmissions. In order to avoid interference between uplink and downlink, for radio network nodes and/or receivers in the same area, uplink and downlink transmissions between radio network nodes and receivers in different cells may be aligned by means of synchronization to a common time reference and use of the same allocation of resources to uplink and downlink.
Thus, when applying FDD, the same numbers of uplink and downlink subframes are available during a radio frame, why HARQ feedback may be provided in an uplink subframe for each received downlink subframe and vice versa. In other words, every downlink subframe can be associated to a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e. to each uplink subframe is associated exactly one downlink subframe. However, in TDD the number of uplink and downlink subframes may be different in some configurations, for example comprising more downlinks subframes than uplink subframes, as illustrated in FIG. 1A.
Generally, one HARQ message is associated with each downlink subframe, since a data packet (e.g., transport block in LTE) is transmitted in one subframe. This implies that HARQ messages from multiple downlink subframes may need to be transmitted in a single uplink subframe, which requires allocation of multiple unique uplink resources for HARQ. In such scenario, comprising e.g. four downlink subframes for each uplink subframe, the receiver has to provide HARQ feedback for all the four downlink subframes in one single uplink subframe, as illustrated in FIG. 1B. When doing so, the HARQ feedback may occupy a significant amount of the uplink communication resources. Hence, in particular for TDD, where an uplink subframe may comprise HARQ messages for many users and from multiple subframes, it is essential that the network nodes can make an efficient uplink resource assignment. This becomes particularly important when there are fewer uplink subframes than downlink subframes in a radio frame, since the amount of reserved uplink control channel resources impacts the available resources for data transmission.
A further constraint for TDD is that a unique allocation is needed for uplink resources that are associated with downlink data transmissions in different downlink subframes.
It is thus a problem to allocate uplink transmit resources for HARQ feedback in a TDD system, such that resources are unique for different subframes while minimizing the uplink resource overhead.
In LTE, the smallest time-frequency entity that may be used for transmission is referred to as a Resource Element (RE), which may convey a complex-valued modulation symbol on a subcarrier. A Resource Block (RB) comprises a set of REs (e.g., 7*12=84 REs) and is of 0.5 ms duration (e.g., 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). The transmission bandwidth of the carrier is divided into a set of RBs. Typical LTE carrier bandwidths correspond to 6, 15, 25, 50, 75 and 100 RBs. Data may be transmitted in the downlink on the Physical Downlink Shared Data Channel (PDSCH), which is scheduled by an associated Physical Downlink Control Channel (PDCCH). The PDCCH is first detected and contains information on the transmission format of the PDSCH. Each transmission of user data (i.e., a transport block) on the PDSCH is performed over 1 ms duration (which is also referred to as a subframe) on one or several RBs and a radio frame consists of 10 subframes.
OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM is a Frequency-Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, Digital Subscriber Line (DSL; originally: Digital Subscriber Loop) broadband internet access, wireless networks, and 4G mobile communications.
In the LTE system, HARQ feedback may be transmitted on the Physical Uplink Control Channel (PUCCH). Here, the notion of PUCCH resource relates to a transmit sequence, a modulation symbol (such as e.g., a Quadrature Phase-Shift Keying (QPSK) or Binary Phase-Shift Keying (BPSK) complex-valued symbol) and a time-frequency resource (e.g., a set of REs or RBs). In general, the more PUCCH resources that are needed for HARQ feedback the less available time-frequency resources for transmitting data on the uplink. In some instances, there is a need to transmit multiple HARQ messages in a subframe, e.g., for TDD as explained above. Another example is for downlink carrier aggregation, where the receiver/UE is simultaneously receiving data from multiple carriers and where independent HARQ messages should be fed back for each carrier. However, the LTE system does not allow HARQ feedback by simultaneous transmission on multiple PUCCH resources since that requires large power back-off in the receiver/UE which causes smaller coverage and worse reliability. Instead, multiple HARQ messages may be fed back on a single PUCCH resource by means of so called channel selection which implies that a set of multiple PUCCH resources (i e., channels) is allocated to the receiver/UE and HARQ information from multiple messages is encoded by which PUCCH resource the receiver/UE selects from the set, together with the modulation symbol transmitted on the selected resource. Channel selection is applicable in LTE for TDD with and without carrier aggregation and for FDD with carrier aggregation.
One type of PUCCH resource allocation for TDD with channel selection is based on implicit reservation. That is the PUCCH resources are not signalled to the receiver/UE, but the receiver/UE determines the resources implicitly from the downlink control channel that schedules the downlink data (PDCCH). This is possible since the PDCCH may be transmitted on one or several Control Channel Elements (CCEs), which denote a set of time-frequency resources. Each PDCCH occupies a unique set of CCEs and a one-to-one mapping may therefore be made from an occupied CCE (the index of the first CCE of the PDCCH is used) to a PUCCH resource, without any resource conflict. An advantage of this is that uplink resources are only reserved when data is transmitted and HARQ feedback is expected, while avoiding introducing uplink resource signalling in the downlink control channel.
As a further development of the LTE system, an Enhanced PDCCH (EPDCCH) is supported. The EPDCCH structure is fundamentally different from that of the PDCCH, e.g., it is based on receiver-specific demodulation reference signals instead of cell-specific reference signals. This enables the use of precoded (i.e., receiver-specifically beam formed) reference signals. While the PDCCH is defined (and can thus be transmitted) over the whole system bandwidth, the EPDCCH may be confined to a configurable receiver-specific set of RBs (i.e., EPDCCH set) and the receiver/UE may be configured with multiple EPDCCH sets. This enables that inter-cell interference coordination could be performed for the EPDCCH by arranging disjoint EPDCCH sets in different cells. Each EPDCCH set comprises a group of (e.g., 2, 4 and 8) Physical Resource Block (PRB) pairs and each PRB pair comprises a set of (e.g., 16) Enhanced Resource Element Groups (EREGs). In turn, the set of EREGs in a PRB pair constitute Enhanced CCEs (ECCEs). The number of ECCEs per PRB pair may typically be 2 or 4 (i.e., corresponding to 8 and 4 EREGs, respectively), depending on the subframe type, i.e., it may be time-varying. Depending on the radio link conditions, an EPDCCH may be transmitted on a set of ECCEs, e.g., 1, 2, 4, 8, 16 or 32 ECCEs, located either within one or a few PRB pairs (i.e., localized transmission), or on all PRB pairs of the EPDDCH set (i.e., distributed transmission). The more ECCEs (or EREGs) that are used to transmit the EPDCCH, the more robust the control channel transmission becomes since a lower code rate could be utilized. The ECCEs are enumerated per each EPDCCH set. The EPDCCH also supports Multi-User Multiple Input Multiple Output (MU-MIMO), such that several EPDCCHs could be transmitted on the same set of ECCEs using different antenna ports.
For localized transmission, an EPDCCH would occupy all EREGs associated with the ECCEs designated for its transmission. However, for distributed transmission, an EPDCCH may not utilize all EREGs of an ECCE in a PRB since it may be transmitted over several PRB pairs and use only a few EREGs per PRB pair. For example, distributed transmission may use 4 EREGs from 4 different PRB pairs (1 EREG per PRB pair), instead of 4 EREGs corresponding to 1 ECCE in 1 PRB pair for localized transmission.
For TDD, the frame structure comprises, in addition to normal subframes, special subframes which contain a first part for downlink transmissions; Downlink Pilot Time Slot (DwPTS), a second part for Guard Period (GP) and last part for uplink transmissions; Uplink Pilot Time Slot (UpPTS), see FIG. 2A. The duration of the different parts may vary and may be configurable by the system.
A downlink subframe is illustrated in FIG. 2B and an uplink subframe is illustrated FIG. 2C.
Moreover, in the LTE system, there is a downlink control region where control channels (e.g., PDCCH) may be transmitted in the 3 (or 4, for small system bandwidths) first OFDM symbols of the subframe, see FIG. 2B. In the special subframes for TDD, the downlink control region is shorter than in normal subframes.
TDD is an attractive multiplexing method as it allows flexible allocation of the resources to uplink or downlink, depending on deployment scenario and traffic load. The EPDCCH has several attractive features that makes it useful in terms of downlink inter-cell interference coordination and beamforming. It would therefore be an advantage if the system could operate with the EPDCCH and in TDD mode. However, it must then be assured that efficient HARQ feedback can be arranged; otherwise the overhead in the uplink control channel will be a significant drawback.
Hence, it is a general problem to assure that there is a reasonable trade-off between control channel overhead and performance.